The invention concerns a tubular filter element for the filtration of fluids, which is arranged in an enclosing housing supplied with at least one inlet and outlet for a to-be filtered fluid wherein the fluid flow is generally inward radially. Caps are provided at both ends to avoid leakage with the housing outlet in such a manner that the fluid path takes place only through the filter medium. The filter element is formed of a flat filter blank consisting of several layers, which is pleated into a plurality of essentially radially extending folds and the folded blank is shaped into a tubular unit in such a manner that the fold webs and fold openings extend in a direction of the inner and outer tube geometry. In the present inventive modification the end folds of the filter bank for the formation of the end fold portion of each end is a web and is directed radially to an inner shell and are connected tight against leakage one with the other, i.e. the end fold portions are connected.
Tubular filter elements are used to filter liquids which may be in the form of suspensions, dispersions, emulsions, colloidial solutions or gases in industry or in laboratory. Such filters have generally gained popularity because of their compact mode of construction and at the same time provide a large filter surface. Since it must be made certain that the fluid path takes place only through the actual filter medium, apart from the sealing at the end caps, the connectable end fold of both the end portions of the flat filter blank forms one of the critical points.
In a known filter element of the initially named mode of construction (GB-PS No. 750 396), the flat filter blank consists of a porous filter support, a layer of filter material of paper and a further filter support. The abutting or near abutting surfaces of the end fold portions are separated one from the other by a radially aligned filter strip and the webs of the end folds portions which are kept separate one from the other, are enclosed by an encompassing clamp of filter material, which is U-shaped in cross-section, is directed in the direction of an inner collecting shell and which is connected tight against leakage through adhesion with the end webs of the flat filter blank. It is also known to connect a filter element, which is constructed in several layers and in which the end webs are directed radially towards the inner collecting shell and somewhat shortened relative to the inner collecting shell, tight against leakage with a thermoplastic synthetic material. In that prior art case, the synthetic material is introduced into a gutter-like end fold so that the longitudinal edges of the webs and the ends, mutually facing and pointing to the inner shell, of the neighbouring folds are connected by thermoplastic or thermosetting synthetic material. This presupposes on the one hand that the space between the end folds is sufficiently large in order to introduce the adhesive substance; on the other hand, the end folds must not splay apart, since clamping must otherwise be employed. This construction has the disadvantage that the high liquid pressure that may be employed lies fully on such synthetic material adhesive bead which holds the filter material together at three sides in an end seam. This manner of end seam connection entails chemical and thermo-physical restrictions and is not suitable for all filter materials. A similar state of the art is known (DE-AS No. 24 17 551, U.S. Pat. No. 3,867,294), where a construction of the end fold is supplied with a synthetic resin bead which terminates with an outer shell and holds together the longitudinal edges, pointing in the direction of the outer shell, of the filter webs and its neighbouring folds. Also in this case, the entire liquid pressure lies on this glued seam. Such a glued seam, be it through gluing or through a thermoplastic synthetic material bead or welding, frequently leads to leakage, since the flat filter blank laid into folds is subject during the filtration operation to movements by reason of pressure differences or by reason of thermal stressing (multiple substance system, thermophysical effects). This applies also in view of the fact that filter elements of that kind are repeatedly subjected to a heat treatment in autoclaves. The thermo-mechanical stressings act radially as well as also in a circumferential direction of the tubular filter element.
It is also already known (U.S. Pat. No. 3,570,675) to connect the fold webs, pointing to an outer shell, of the end folds in the end region or also over the entire length by an adhesive substance arranged therebetween. Such a connection is usual particularly in the case of single layer filter material. Through the U.S. Pat. No. 4,184,966, it is also known in a multi-layer flat filter blank to let the webs end radially in direction of an outer shell and to connect both the webs by means of a rigid strip, having a U-shape in cross-section, of thermpolastic synthetic material in such a manner that the legs of the U-shaped thermoplastic strip each engage two layers of the mutually facing webs and are thermoplastically welded together with them. The web of the U-shaped blank in that case points in the direction of the outer shell of the filter element. Apart from the difficulties in the manipulation (stresses in the pleating cylinder, risk of injury of the membrane, more difficult introduction of the U-shaped strip), this seam formation also has the disadvantage that the entire fluid pressure lies perpendicularly from the outer shell on the longitudinal edges of the webs of the end folds so that the risk exists in such a case that these or individual layers detach from the thermoplastic strip and unifiltered medium thereby migrates to the filtrate side. A further disadvantage of such a seam formation is to be seen in that all layers of the flat filter blank must be thermoplastically activatable or thermally loadable in order that a connection one among the other and with the U-shaped thermoplastic strip is possible. Furthermore, it is disadvantageous that the end fold as seen from the inner shell does not have the same freedom of movement as the neighboring folds. The construction of the end fold according to the U.S. Pat. No. 4,184,966 is not always possible without removing the end webs of non-thermoplastic materials.